1. Field of the Invention
The present invention relates to a method and instrument for NMR (nuclear magnetic resonance) measurements. More particularly, the invention relates to an NMR measurement method and NMR pulse sequence for observing plural pathways and correlations simultaneously in multidimensional NMR spectroscopy.
2. Description of Related Art
In almost all multidimensional NMR measurements generally conducted heretofore, a certain one pathway is magnetized and transferred. Then, the magnetization is observed as a signal. That is, at the beginning of the experiment, a magnetization of some nucleus is excited and moved to another nucleus. Finally, the magnetization is brought to an observable nucleus.
In other measurements, there are plural magnetization transfer pathways to be measured. Some techniques of observing signals consisting of magnetizations made to follow plural pathways in this way have been reported. The method closest to the present invention is disclosed in Yuxi Pang et al., “High-resolution of five frequencies in a single 3D spectrum: HNHCACO-a Bidirectional Coherence Transfer Experiment”, J. of Biomol. NMR, 11, 185-190 (1998) where signals of magnetizations made to follow two pathways through a protein are observed. The pulse sequence used is illustrated in FIGS. 9A to 9F. This method is a three-dimensional experiment performed to obtain chemical shifts of five nuclei 1HN, 15N, 13Cα, 13CO, and 1Hα of the backbone of a protein or peptide.
This experiment is so set up as to observe an NMR signal consisting of two magnetization transfer pathways in the backbone of a protein or peptide. FIG. 10 shows two magnetization transfer pathways through a protein or peptide structure formed by peptide coupling of two amino acid residues.
Pathway 1 for magnetization of excited 1HN consists of chemical shift evolution (t1) at 15N, chemical shift evolution (t2) at 13CO, transfer of the magnetization to 13Cα, and observation (t3) at 1Hα as given below. A chemical shift evolution referred to herein is a phenomenon in which the sense of magnetization becomes nonuniform due to chemical shift (deviation in frequency from the rotational frequency of a rotating system of coordinates).
Pathway 1: 1HN→15N (t1)→13CO (t2)→13Cα→1Hα (t3)
Pathway 2 for magnetization of excited Hα consists of chemical shift evolution (t1) at 13Cα, chemical shift evolution (t2) at 13CO, transfer of the magnetization to 15N, and observation (t3) at 1HN as given below.
Pathway 2: 1Hα→13Cα(t1)→13CO (t2)→15N→1HN (t3)
The problem with this experiment is that the pulse sequence is lengthened due to execution of plural chemical shift evolutions. Hence, the sensitivity is deteriorated by the effects of magnetization relaxation.
This problem is hereinafter described. In period t1 of this experiment, the magnetization in the pathway 1 is oriented in the Z-direction to prevent chemical shift evolution. During this time interval, a chemical shift evolution 13C (t1) in the pathway 2 is carried out. Then, the magnetization in the pathway 2 is oriented in the Z-direction to prevent chemical shift evolution. During this time interval, a chemical shift evolution 15N (t1) in the pathway 1 is carried out. Since the chemical shift evolution during each t1 and the chemical shift evolution during each t2 are each made up of a long pulse sequence (such as tens of milliseconds), it takes a long time until a detection is started. The magnetization returns to a steady state with elapse of time. In this case, the magnetization oriented in the Z-direction is not time-modulated by chemical shifts but a spin-lattice relaxation of the magnetization occurs. Therefore, in practice, the efficiency is very low and the sensitivity is poor although the measurement is being performed. The signal sensitivity obtained finally decreases.
For this reason, where magnetizations are being observed generally in the same pathway using a pulse sequence, an NMR signal can be observed with better sensitivity if the pulse sequence is shortened because the effect of relaxation is reduced.
On the other hand, in multidimensional NMR spectroscopy, the pulse sequence can be shortened by the concatenation technique by simultaneously carrying out mutual transfer of in-phase magnetization and antiphase magnetization and chemical shift evolution. For example, during generally used chemical shift evolution of 15N in a HNCO experiment, transfer of magnetization from antiphase magnetization of 15N and 13CO to in-phase magnetization of 15N is effected while performing a chemical shift evolution of 15N. Thus, the pulse sequence is shortened. Furthermore, general multidimensional NMR methods with three-dimensionality or higher, such as the aforementioned HNCO experiment and CBCA(CO)NH experiment, the concatenation technique is effectively utilized. In each method, however, plural independent evolution periods are necessary. Consequently, a measurement is performed for a long time for evolution as each different axis for multidimensional measurement.
In the aforementioned experiment by Pang et al., magnetization is transferred in two directions simultaneously. In this experiment, the pulse sequence itself is lengthened. Therefore, the sensitivity loss due to relaxation is increased. Furthermore, in general multidimensional NMR spectroscopy with three-dimensionality or higher including the method of Pang et al., the measurement is composed of plural indirect chemical shift evolution periods and so there is the problem that a long duration measurement is necessary.
As described so far, the measurement method consists of generally using transfer of magnetization along one pathway. It is only possible to measure a signal arising from magnetization passed through the single pathway. In order to obtain information about plural pathways, it has been necessary to conduct a separate experiment for each pathway.
Furthermore, in the technique of simultaneously observing plural pathways using a special measurement method as described in the above-cited reference, chemical shift evolution of plural nuclei needs to be performed in separate evolution periods. Since the loss of sensitivity due to relaxation of nuclear magnetization is large, it has been rare that useful measurement results are obtained in practice.
Additionally, in order to obtain plural chemical shifts, they need to be measured as respective axes. Hence, a long duration measurement is required to perform such a multidimensional measurement. As a result, the measurement sensitivity-per-unit time has been low.